It is known that during a heavy braking maneuver while driving a vehicle, the vehicle occupants may move forward in their seats relative to the vehicle, due to their momentum, before they are restrained by their seat belts. In such events, occupant movement relative to the vehicle is arrested only after the upper body of the occupant has moved away from the backrest of the vehicle seat.
In some events, where the vehicle is decelerating quickly, the movement of the upper body against a locked seat belt may generate a load of 400 N or more in the restraint device. This may be uncomfortable for the vehicle occupants, and to mitigate these loads, active seat belt tensioners have been developed. Active seat belt tensioners use an actuator such as an electric motor to apply a tensioning force to a spool around which the seat belt webbing is wound.
With a conventional inertia seat belt, the spool is arranged to freely pay out seat belt webbing until vehicle acceleration in the longitudinal direction exceeds a pre-determined threshold, at which point a spool lock is activated, preventing further pay out of webbing from the spool. In the event the vehicle driver starts to brake heavily, there is a delay between the application of the vehicle brakes and the locking of the seat belt spool. In this case, the occupants will be moving forward away from their seat backs, until the pay out of webbing is arrested and the occupant is restrained by the locked seat belt. At the point where the moving occupant is restrained by the locked belt, the occupant will feel a sudden increase in the force applied by the webbing restraining them, which may be perceived as uncomfortable.
In contrast, in the event the vehicle is fitted with active seat belt tensioners, the tensioner motor applies a light tensioning force to the webbing spool as vehicle deceleration is detected exceeding a pre-determined threshold, greatly reducing the pay out of webbing and even retracting belt slack, before the occupant is restrained by the seat belt. Such devices are intended only to reduce slack in the seat belt system and mitigate the restraint forces felt by the occupants from the seat belts during normal driving maneuvers such as braking.
Typical active seat belt tensioner devices have a retraction force of around 200 N; this is sufficient to reel-in the majority of any slack in the seat belt webbing before the occupant is restrained thereby. It will be appreciated by one skilled in the art that restraint performance and comfort of the occupant can be greatly improved if the seat belt webbing is tensioned appropriately prior to the seat belt restraining the occupant during vehicle deceleration.
While these active seat belt tensioner devices are generally successful in mitigating the restraint forces felt by the occupants during heavy braking, when compared to standard inertia seat belt devices, synchronisation of their activation with the behaviour of the vehicle is critical if their activation is to be almost imperceptible and not become a distraction to the occupant. Due to their momentum, however, occupants typically move forward slightly in their seats before vehicle deceleration exceeds the triggering threshold. Owing to this relative movement between the occupant and the vehicle, known active seat belt tensioner devices, which are triggered by measured vehicle deceleration, must apply a tensioning force quickly to the seat belt. This sudden application of force to the seat belt is perceivable to the occupants in some normal driving conditions and may be undesirable.
What is required is a method and means of triggering an active seat belt tensioner in advance of the occupant moving away from their seat back.
Ideally, an active seat belt tensioner device would enhance restraint comfort by activating according to driver demand rather the response by the vehicle to said demand. An ideal seat belt tensioner device would remove all slack in the seat belt system in a manner imperceptible to the user, coupling them to the vehicle as early as possible during braking and minimising peak restraint force.
Preferably an active occupant restraint is adapted to be tuned to permit a vehicle occupant to be aware of the operation of the active device(s) without such operation being a distraction—for example by application of pre-determined maximum seat belt tension at maximum retraction speed under all triggered conditions. Alternatively operation of the active device(s) may be substantially imperceptible to the occupants.
A trigger for an active device could be based on rising fluid pressure in the vehicle braking system exceeding a pre-determined pressure threshold, but is necessarily delayed by the initial take-up of slack in the brake pedal actuation linkage and by the initial forward movement of the usual master cylinder piston over a recuperation port. Such delay may provide sufficient time for the upper body of an occupant to leave the vehicle seat back before the seat belt can be tensioned.
On the other hand, a trigger could be based on brake pedal movement in order to improve synchronization of the active device with occupant behaviour. However this may be irritating to the driver, since the control system of a corresponding seat belt actuator may be unable to distinguish between a light touch to modulate vehicle speed and a firm brake pedal application for an emergency stop.
Accordingly the means of actuating the active device should not be susceptible to false inputs.
It is an aim of the present invention to address one or more of these issues. Other aims and advantages of the invention will become apparent from the following description, claims and drawings.